A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor formed using amorphous silicon has low electric field effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a thin film transistor formed using crystalline silicon has high electric field effect mobility, but a crystallization process such as laser annealing is necessary and such a transistor is not always suitable for a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is formed using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.